The present invention relates to fusible polyimide, a fusible polyimide film, a composite polyimide film having a fusible surface, and a metal film/composite polyimide film laminate, and a metal film/composite polyimide film/metal film laminate.
Aromatic polyimide films show good high temperature resistance, good chemical properties, high electrical insulating property, and high mechanical strength, and therefore are widely employed in a variety of technical fields. For instance, an aromatic polyimide film is favorably employed in the form of a continuous aromatic polyimide film/metal film laminate structure for manufacturing a flexible printed circuit board (FPC), a carrier tape for tape-automated-bonding (TAB), and a tape of lead-on-chip (LOC) structure.
The aromatic polyimide film/metal film laminate can be produced by bonding a polyimide film to a metal film using a conventional adhesive such as an epoxy resin. However, due to low heat-resistance of the conventional adhesive, the produced composite sheet cannot show satisfactory high heat-resistance.
For obviating the above-mentioned problem, a variety of bonding methods have been proposed. For instance, an aromatic polyimide film/metal film laminate sheet is manufactured by producing a copper metal film on an aromatic polyimide film by electro-plating. Otherwise, an aromatic polyamide solution (i.e., a solution of a precursor of an aromatic polyimide resin) is coated on a copper film, dried, and heated for producing an aromatic polyimide film on the copper film.
An aromatic polyimide film/metal film composite sheet also can be produced using a thermoplastic polyimide resin.
U.S. Pat. No. 4,543,295 describes that a metal film/aromatic polyimide film laminate in which the metal film is bonded to the polyimide film at a high bonding strength is prepared by combining, by heating under pressure, a metal film and a composite aromatic polyimide film composed of a highly heat resistant substrate film and a thermoplastic thin polyimide layer bonded to the substrate film at a high bonding strength.
Each of Japanese Patent Publication No. 7-102648 and Japanese Patent Provisional Publication No. 10-138,318 describes a composite polyimide film produced by a co-extrusion method.
It is an object of the invention to provide fusible aromatic polyimide which can be bonded to a metal film, particularly, a stainless steel film and a copper film, with a high strength and which can be easily etched by a chemical etching method (or wet etching method).
The invention resides in fusible polyimide showing a fusion endothermic peak in a differential scanning calorimeter (DCS), preferably at a temperature of 340-380xc2x0 C., which has a recurring unit of the formula (1): 
in which Ar1 is a mixture of residues of tetracarboxylic dianhydrides comprising 12-25 mol. % of a residue of pyromellitic dianhydride, 5-15 mol. % of a residue of 3,3xe2x80x24,4xe2x80x2-benzophenonetetracarboxylic dianhydride, and a remaining mol. % of a residue of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride, and Ar2 is an aromatic diamine residue comprising 1,3-bis (4-aminophenoxy)benzene.
The invention also resides in a fusible polyimide film showing a fusion endothermic peak in a differential scanning calorimeter, preferably at a temperature of 340-380xc2x0 C., which comprises aromatic polyimide having a recurring unit of the aforementioned formula (1).
The invention further resides in a composite polyimide film comprising a polyimide substrate sheet and a fusible polyimide film united to the substrate sheet, in which the fusible polyimide film shows a fusion endothermic peak in a differential scanning calorimeter, preferably at a temperature of 340-380xc2x0 C., and comprises aromatic polyimide having a recurring unit of the formula (1).
The invention further resides in a metal/composite polyimide film laminate comprising the above-mentioned composite polyimide film and a metal film combined to the composite polyimide film via the fusible polyimide film.
The invention furthermore resides in a metal/composite polyimide film/metal laminate comprising a first metal film having a thickness of 3 to 35 xcexcm, a composite polyimide film having a thickness of 7 to 150 xcexcm, and a second metal film having a thickness of 30 to 200 xcexcm, in which the composite polyimide film comprises a polyimide substrate sheet and a fusible polyimide film united to the substrate sheet, in which the fusible polyimide film shows a fusion endothermic peak in a differential scanning calorimeter and comprises aromatic polyimide having a recurring unit of the aforementioned formula (1).
The invention furthermore resides in a metal film/composite polyimide film/metal film laminate in which each metal film independently is a copper film or a stainless steal film, comprising a first metal film having a thickness of 3 to 35 xcexcm, a composite polyimide film having a thickness of 7 to 150 xcexcm, and a second metal film having a thickness of 3 to 200 xcexcm, in which the composite polyimide film comprises a polyimide substrate sheet and a fusible polyimide film united to each surface of the substrate sheet, in which the fusible polyimide film shows a fusion endothermic peak in a differential scanning calorimeter, the composite polyimide film shows a modulus in tension in the range of 400 to 1,000 kgf/mm2 at 25xc2x0 C. in machine direction and traverse direction, the composite polyimide film shows a chemical etchable rate of 2.0 xcexcm/min. or more, and both of the metal films are bonded to the composite polyimide film at 90xc2x0 peel strength of 0.8 kgf/cm or more.
The chemical etchable rate is determined by the following procedure:
A specimen (i.e., laminate of a metal film/polyimide film/metal film, thickness: 20 xcexcm) is immersed in an etching solution (composition: 36 wt. % of potassium hydroxide, 37 wt. % of monoethanol amine, and 27 wt. % of water) heated to 80xc2x0 C. Thereafter, a period of time required for completely dissolving the polyimide film in the etching solution is measured.